[IMP] base: better help message on report fields
[IMP] base: better help message on report fields

NULL is too technical
Courtesy of Lionel Sausin

stephen144/odoo,Elico-Corp/odoo_OCB,hip-odoo/odoo,microcom/odoo,bplancher/odoo,microcom/odoo,sysadminmatmoz/OCB,stephen144/odoo,bplancher/odoo,hip-odoo/odoo,laslabs/odoo,microcom/odoo,hip-odoo/odoo,sysadminmatmoz/OCB,stephen144/odoo,dfang/odoo,sysadminmatmoz/OCB,syci/OCB,optima-ict/odoo,optima-ict/odoo,laslabs/odoo,hip-odoo/odoo,ygol/odoo,storm-computers/odoo,laslabs/odoo,sysadminmatmoz/OCB,Elico-Corp/odoo_OCB,sysadminmatmoz/OCB,bplancher/odoo,ygol/odoo,syci/OCB,stephen144/odoo,microcom/odoo,syci/OCB,dfang/odoo,ygol/odoo,optima-ict/odoo,storm-computers/odoo,storm-computers/odoo,syci/OCB,stephen144/odoo,bplancher/odoo,sysadminmatmoz/OCB,ygol/odoo,ygol/odoo,syci/OCB,dfang/odoo,optima-ict/odoo,storm-computers/odoo,Elico-Corp/odoo_OCB,optima-ict/odoo,storm-computers/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,microcom/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,dfang/odoo,sysadminmatmoz/OCB,bplancher/odoo,dfang/odoo,optima-ict/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,syci/OCB,storm-computers/odoo,dfang/odoo,microcom/odoo,stephen144/odoo,bplancher/odoo,ygol/odoo,laslabs/odoo,Elico-Corp/odoo_OCB